In general, in the case of overprinting a multicolor original (original for multicolors) with a plurality of color inks, so-called “color separation” is performed. In the color separation, multivalued pixel values of a color space (for example, RGB color space) corresponding to original colors (color lights) in a multicolor original image are converted into multivalued pixel values of a color space (for example, CMY color space) corresponding to color inks used for printing.
An algorism of color separation processing is broadly divided into a method of converting the RGB color space into the CMY color space by a transformation represented by Neugebauer's equation (for example, Japanese Patent Laid-Open Publication No. 2001-277473) and a method of converting the RGB color space into the CMY color space by using a color conversion lookup table (hereinafter, abbreviated as “color conversion LUT”) (for example, Japanese Patent Laid-Open Publication 2001-186368). The algorism of the color separation processing by the color conversion LUT is more often used, in which it is easy to perform an intentional shift of a color gamut (preferable color reproduction).
In the case of performing the overprinting with the plurality of color inks, specifically, before a color ink to be previously printed (hereinafter, referred to as “previously-printed color ink”) is dried up, a color ink printed later (hereinafter, referred to as “later-printed color ink”) is overprinted thereon. Accordingly, problems such as a bleeding phenomenon, a trapping phenomenon, and a back-trapping phenomenon occur between the previously-printed color ink and the later-printed color ink. Therefore, a desired result of the printing cannot be obtained.
Note that the bleeding phenomenon is a phenomenon that the previously-printed color ink and the later-printed color ink are blended, the trapping phenomenon is a phenomenon that the later-printed color ink is transferred onto the previously-printed color ink, and the back-trapping phenomenon is a phenomenon that the previously-printed color ink is absorbed to an image carrier for the later printing (equivalent to a printing drum in a stencil printing machine according to the present invention). Among them, influences from the trapping phenomenon and the back-trapping phenomenon on the result of the printing cannot be ignored.
Accordingly, it has been necessary to prepare the color conversion LUT with the trapping phenomenon and the back-trapping phenomenon taken into consideration. However, the influences from the trapping phenomenon and the back-trapping phenomenon are varied owing to the color inks used for overprinting and an overprint sequence of the color inks. Accordingly, it has been necessary to prepare the color conversion LUTs corresponding to the respective cases.
Therefore, enormous cost and time have been required for creating the color conversion LUTs, and a storage capacity of a storage device which stores the color conversion LUTs has been enormously increased.